leftwm: remove unused patch file

As of the upgrade in https://github.com/NixOS/nixpkgs/pull/82188, we no longer
need the patch file; I meant to delete it in the previous PR but forgot.

pkgs/applications/window-managers/leftwm/cargo-lock.patch

@@ -1,13 +0,0 @@
-diff --git a/Cargo.lock b/Cargo.lock
-index 915ab04..3d5956d 100644
---- a/Cargo.lock
-+++ b/Cargo.lock
-@@ -370,7 +370,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
-
- [[package]]
- name = "leftwm"
--version = "0.1.9"
-+version = "0.1.10"
- dependencies = [
-  "bytes 0.4.12 (registry+https://github.com/rust-lang/crates.io-index)",
-  "chrono 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",